1. Field
The invention relates to the field of communications. More specifically, the invention relates to a switching device and method for policing the flow of information over a network.
2. General Background
Computer networks provide a mechanism for transferring information between various locations. One type of computer network, referred to as an Ethernet, is a packet-based local area network (LAN) using Carrier Sense Multiple Access with Collision Detect (CSMA/CD) access protocol. Typically, Ethernet is configured in accordance with Open System Interconnection (OSI) and the Institute of Electrical and Electronic Engineers (IEEE) 802.3 standard.
As shown in FIG. 1, an embodiment of the seven-layer OSI reference model 100 for a conventional Switched Ethernet is shown. Switched Ethernet comprises a physical layer 110, a data link layer 120, a network layer 130, a transport layer 140, a session layer 150, a presentation layer 160 and an application layer 170. More specifically, physical layer 110 is concerned with the transmission of bits across a medium (e.g., twisted pair, coaxial, optical fiber, etc.) while data link layer 120 is concerned with the point-to-point connection between two devices (e.g., switches, computers, etc.). As shown, data link layer 120 is separated into two sublayers, one of which is a media access control (MAC) sublayer 121. MAC sublayer 121 not only arbitrates for access to the medium using its unique MAC address, but also produces data frames for subsequent transmission as a stream of bits over the medium.
Referring still to FIG. 1, network layer 130 is concerned with the transmission of data across multiple links or multiple networks connected by switches. For conventional Switched Ethernet, a policing function 180 is employed within network layer 130 or a higher level layer (e.g., transport). Policing function 180 enforces xe2x80x9ctraffic contractsxe2x80x9d established by differentiated services for example (described below).
As set forth in an Internet Draft of the Internet Engineering Task Force (IETF) entitled xe2x80x9cDifferentiated Servicesxe2x80x9d published on or around February 1999, differentiated services provides an architecture from which Internet Service Providers (ISPs) can offer a range of network services at different pricing to each customer. More specifically, a customer selects a particular level of service at a predetermined price. Upon selecting a service level, the customer has entered into a traffic contract with the ISP. The xe2x80x9ctraffic contractxe2x80x9d involves the ISP agreeing to support a particular transmission bit rate associated with the chosen service level (referred to as the xe2x80x9cnegotiated bit ratexe2x80x9d) in exchange for monetary compensation. Residing in network layer (OSI layer 3) 130 for example, policing function 180 attempts to enforce the traffic contract. However, the latency realized in routing data from physical and data link layers 110 and 120 to network layer 130 prevents strict enforcement of the traffic contract.
As shown in FIG. 2, an illustrative embodiment of a conventional Switched Ethernet 200 is shown. Switched Ethernet 200 comprises a switch 210 that includes a finite amount of buffer memory 215. In this embodiment, switch 210 operates as a multiport device having N addressable ports 2201-220N (xe2x80x9cNxe2x80x9d is a positive whole number). A plurality of computers 230 are coupled to a portion of addressable ports 2201-220N via a plurality of links 240. This supports simultaneous, half or full-duplex transmissions of frames between computers 230 and addressable ports 2201-220N of switch 210. A xe2x80x9cframexe2x80x9d includes one or more packets of data, address and/or control information arranged in a predetermined format. In addition, switch 210 is coupled to a link 250.
It has been appreciated that Switched Ethernet may be implemented with different varieties of links. For example, link 250 may be a 100BASE-T link that offers a faster transmission rate than links 240 (e.g., 10BASE-T links). In certain situations, where switching device 210 is not able to handle burst transmissions from devices coupled to link 250, network performance may worsen. Of course, this performance degradation could be reduced by adding substantial buffer memory to switch 210 at a substantial increased cost.
Recently, in accordance with IEEE 802.3x, additional features have been added to handle the above-described performance issues for full-duplex Switched Ethernet. For example, once buffer memory 215 is full, switch 210 is capable of throttling the transmission of incoming frames from a computer (e.g., computer 2301) by sending a particular type of Media Access Control (MAC) control frame 260 back to computer 2301. This type of MAC control frame 260 is referred to as a PAUSE frame. Once computer 2301 decodes and recognizes incoming information as a PAUSE frame, computer 2301 pauses the transmission of frames for the specified time period. Once the time period has expired, computer 2301 continues to transmit frames. However, a PAUSE frame 260 is used solely and exclusively to handle overflow conditions of buffer memory 215.
Hence, it would be desirable to develop a switching device and method to utilize data link layer functionality for enforcement of traffic contracts.
The present invention relates to a switching device and method for policing a flow of information over a network. The policing function is situated at the data link layer and capable of receiving a transmission rate from a traffic contract at a higher level. The close proximity of the policing function to the physical medium enables stricter enforcement of traffic contracts.
For full-duplex communications, the policing function uses a PAUSE frame to throttle input. A PAUSE frame is issued when an accumulated count value has exceeded an interval bit rate based on the transmission rate set by a traffic contract. Multiple PAUSE frames may be used to halt transmissions beyond the maximum delay time provided by a single PAUSE frame. For half-duplex communications, the policing function relies on collision-based backpressure and carrier sense backpressure techniques.